In an effort to protect Earth's environment and to conserve natural resources, the reduction of greenhouse gas emissions has been identified as a priority by governments around the world. One way to reduce greenhouse gas emissions is to reduce energy consumption. The consumption of energy can be reduced by using energy efficient devices, including illumination devices.
Optical illumination devices and systems using planar or slab light guides are energy efficient illumination devices. Such illumination devices typically include various layers of optically transmissive layers or media. Conventionally such illumination devices may be divided into a layer that receives and guides light from one or more light sources (in the present application called a light guide layer—but also called a variety of other names in the art) for insertion into a layer that redirects the light (in the present application called a light redirection layer—but also called by a variety of other names in the art) for emission from the illumination device. Depending on the configuration and construction of a particular illumination device, these layers may be areas of a unitarily manufactured structure or may be separate physical structures that have been separately manufactured and subsequently brought together to form a single structure that operates as a unit. Some examples of planar illumination devices are shown in U.S. Patent Application Publication No. US2010/0202142.
Where the layers are manufactured separately and are subsequently assembled to form a unit, the interfaces between the layers inherently present discontinuities in the optical transmission path, such that light being transmitted can be subject to reflection losses, more commonly known as Fresnel losses in the art, particularly when the layers are misaligned. The degree of Fresnel losses affecting the efficiency of light transmission depends on the quality of the optical bond or coupling existing at a given optical transmission interface and the quality of the fabrication of the optical components.
Current conventional planar illumination devices that are highly efficient are therefore not easy to manufacture as extreme precision, both in the fabrication of the layers and in their alignment when they are brought together to form a unit, is required. Small defects either in fabrication or in alignment will have a very significant negative effect on the percentage of light received from the light source that the illumination device is able to emit, and must generally be avoided.
To the extent that optical coupling across the different light transmission materials and layers used in light guide illumination devices can be made more efficient, such as (but not limited to) reducing Fresnel losses, a higher performance product having lowered energy consumption while providing a higher-intensity output, can be achieved.